This invention relates generally to metallurgical apparatus, and particularly to a crucible for containing mixtures of magnesium and magnesium chloride. The invention also relates to a method for manufacturing such a crucible, to protect it from attack by molten magnesium.
It is common to provide crucibles or chemical reactors with stainless steel linings or layers, to prevent corrosion of the major component of the crucible, which is often carbon steel.
In the production of zirconium and titanium by the Kroll process, a normal byproduct is a mixture of magnesium (Mg) and magnesium chloride (MgCl.sub.2). To improve process economics, it is desirable to separate these two substances so that the magnesium may be recycled in the process. One method of separating magnesium from magnesium chloride is to melt the mixture under an inert atmosphere. The densities of the liquids are different, and when the mixture is fully molten, magnesium floats to the top. The melting temperature of magnesium is about 650.degree. C., while that of magnesium chloride is about 715.degree. C. Thus, the presence of magnesium chloride generated by the Kroll process increases the required crucible temperature. Furthermore, in order to establish reasonably rapid heat transfer to the melt, it is necessary to heat the crucible containing the mixture to about 850.degree. C. Such temperatures not only aggravate any solvent attack by molten magnesium on the crucible (that is, the tendency for the magnesium to dissolve the crucible), but also render unsuitable, from a strength or creep resistance standpoint, certain materials that do not react with molten magnesium, such as carbon steel.
Creep is the slow, plastic deformation that occurs in a metal subject to high temperature and stress over a long period of time. It may be measured in various ways, for example, by percent elongation per unit time at a given applied stress. A bar made of 316 stainless steel subjected to a tensile stress of 3000 psi at a temperature of 1500.degree. F. will grow by about 0.1% in length every 1000 hours; a low carbon 1005 steel under the same conditions will grow much faster.
It is possible to use a crucible constructed entirely from carbon steel, for example SA-516 grade 70, about two inches thick, but under the temperatures indicated, such a crucible distorts and is rendered useless by creep effects in short order. In addition, the exterior surface of the crucible oxidizes severely, and spalls off in large flakes. The spalling reduces the thickness of the crucible in time, and also provides an opportunity for short circuiting the electric furnace in which the crucible is placed. Thus, the temperatures mentioned are too high for mild carbon steels.
U.S. Pat. No. 4,353,535 describes a crucible fabricated from type 444 stainless steel, which is a ferritic stainless steel. This is an improvement over carbon steel crucibles, particularly where only molten magnesium is to be contained. Ferritic stainless steels resist surface oxidation, and possess higher temperature strength and creep resistance than do mild steels, but ferritic stainless steels are still inadequate for the purpose mentioned above, and will suffer similar distortion and failure in time.
Another solution has been to use a crucible machined from graphite. However, some carbon from such crucibles may dissolve into the melt, to the detriment of the process. Furthermore, graphite is expensive, and comparatively fragile.
Austenitic stainless steels, which are characterized by their nickel content, do possess adequate strength and creep resistance at 850.degree. C.; however, steels of this type are attacked by molten magnesium. Apparently, molten magnesium attacks the nickel component, leading to intergranular cracks and ultimate weakening. Therefore, austenitic stainless steel can be used as a crucible for containing 850.degree. C. magnesium mixtures only if the interior of the crucible is coated with a non-reactive metal, such as carbon steel, lacking the nickel component.
It is known to join layers of mild steel and austenitic stainless steel by explosive cladding. However, explosive methods are very expensive, and, because there are few practitioners of this art, it is difficult to have crucibles produced without long production delays.
Lining a stainless steel crucible with carbon steel by more conventional methods, such as weld overlaying, is problematic, because theory predicts that it will not work. When heated, austenitic stainless steel expands at a rate about 30% greater than does mild steel; that is, the coefficients of thermal expansion are substantially different. The differential expansion is so great that, at 850.degree. C., tensile failure (fracturing) of the carbon steel is predicted. Indeed, experts in this field are reluctant to take on the task of overlaying a stainless steel crucible with mild steel.
It should be noted that metals generally fail in tension, not compression. Therefore, prior workers who overlaid carbon steel substrates with relatively thin layers of stainless steel did not encounter cracking problems, since as the laminates were heated, the stainless coating layer was placed in compression when heated (the much thicker substrate receiving being placed in relatively little tension).
We have discovered that, even though fracturing does occur in a mild steel liner deposited on an austentic stainless steel substrate, the fractures do not extend the full depth of the liner. Thus, we have discovered that weld overlaying can be used to protect an austenitic stainless steel liner from molten magnesium at 850.degree. C.